White wine, fermentation

Refrigerating the environment is another way of utilizing cold to stabilize wine. Winter cold may be used for this purpose, as well as specialized vat room installations where a chilled atmosphere makes it possible to keep wine at a temperature in the vicinity of 0°C for one or two months. The wine must previously have been filtered. Vats may be equipped with individual exchangers to accelerate cooling. This type of installation is highly effective. Furthermore, it minimizes the risk of oxidation, as the wine does not need to be handled. This system may also be used for fermenting white wines, maintaining a fermentation temperature of around 18-20°C, or even lower. 

Fig. 1. An amplified outline scheme of the making of various wiaes, alternative products, by-products, and associated wastes (23). Ovals = raw materials, sources rectangles = wines hexagon = alternative products (decreasing wine yield) diamond = wastes. To avoid some complexities, eg, all the wine vinegar and all carbonic maceration are indicated as red. This is usual, but not necessarily tme. Similarly, malolactic fermentation is desired in some white wines. FW = finished wine and always involves clarification and stabilization, as in 8, 11, 12, 13, 14, 15, 33, 34, followed by 39, 41, 42. It may or may not include maturation (38) or botde age (40), as indicated for usual styles. Stillage and lees may be treated to recover potassium bitartrate as a by-product. Pomace may also yield red pigment, seed oil, seed tannin, and wine spidts as by-products. Sweet wines are the result of either arresting fermentation at an incomplete stage (by fortification, refrigeration, or other means of yeast inactivation) or addition of juice or concentrate.

Although it is sometimes encouraged in white wines, particularly barrel-fermented Chardonnay, this fermentation tends to lower fmitiness and be considered undesirable in other white wines unless acidity is too high. This is also tme for pink and light red wines. If it occurs after bottling, a gassy, cloudy wine results. In such wines, it can be avoided by careful attention to clarification or filtration sufficient to remove the bacteria, by adding SO2 at appropriate intervals as an inhibitor, or by pasteurization. 

Wine. The earliest known wines were made in Iran about 5400—5000 BC (25). The species of grape used is unknown and may have been either the wild grape Fitis viniferus sylvestris or a cultivated precursor of the modem wine grape V. viniferus viniferus. The source of the yeast used, and the procedures used are completely unknown. In modem times, grapes (about 21—23% sugar) are pressed the liquid must is either separated and allowed to settle for 1—2 days (for white wines) before inoculation with yeast, or the whole mass is dkectly inoculated with yeast (for red wines). In either case, while the initial fermentation takes place, the carbon dioxide formed by fermentation excludes ak and prevents oxidation. White wines are transferred to a second fermentor (racked) near the end of fermentation and kept isolated from the ak while solids, including yeast, settle out, a process that requkes about six... 

The optimum temperature of fermentation of wine depends on the type, red wines working best at about 29°C while the white wines require a cooler condition of around 16°C. Heat is given off by the chemical process of fermentation. They are then traditionally matured and stored in caves or cellars at about 10°C. Much of the manufacture... 

The complexity of wine composition is a central reason for the vast variety of wines in the marketplace. In addition to water and ethanol, the major components, a variety of organic acids as well as metal ions from minerals in the skin of the grape are present. Initially, all of these substances remain dissolved in the bottled grape juice. As the fermentation process occurs, the increasing alcohol concentration in the wine alters the solubility of particular combinations of acid and metal ions. Unable to remain in solution, the insoluble substances settle as crystals. Since the process of red-wine making involves extended contact of the grape juice with the skins of the grapes (where the minerals are concentrated), wine crystals are more common in red wines than in white wines. 

White musts and wines made without maceration contain very low amounts of flavonoids. However, when making white wine from white grapes, skin contact at low temperature is sometimes performed before pressing and fermentation to increase extraction of volatile compounds and aroma precursors. After 4h of skin contact, the concentration of flavanol monomers and dimers in must was increased threefold. Delays between harvest and pressing, especially if sulfur dioxide is added to prevent oxidation, as well as thorough pressing, similarly result in increased concentrations of flavonoids in white musts and wines. " " ... 

The manner in which yeast contributes to the fermentation process was not clearly understood until 1857, when the French microbiologist Louis Pasteur discovered that not only does the fermentation process require any oxygen, but also alcohol yield is actually reduced by its presence. The amount of ethanol generated by this first alcoholic fermentation is about 11%. At this step, "champagne" is still actually a noneffervescent white wine, because the carbon dioxide produced during the first alcoholic fermentation is allowed to escape into the atmosphere. 

Figure 8.9—Separation of the main organic acids in white wine. Malic and lactic acids are indicators of the classical malolactic (fermentation process application note from TSP).

The rate at which the M-L fermentation proceeds is a function of the generation time. As pH and temperature decrease, the generation time increases, and the M-L fermentation proceeds more slowly. These observations agree with the conclusions of Bousbouras and Kunkee (65) in relation to the effect of pH on the rate of M-L fermentation. Even at the low pH values observed in some of the native white wines, Leuco-nostoc organisms were able to develop and complete the M-L fermentation. The cool cellar temperatures did not inhibit the M-L fermentation... 

White wines are more prone to darkening from excessive enzymatic or oxidative browning, and all wines are difficult to clarify following fermentation if proper care is not given in the crushing and destemming operation. 

Chemical Analysis. It is suggested that a set procedure be established for running specific analyses according to a specific timetable. For example, a visual and taste inspection of all wines every two weeks (two months for wines in barrels), malic and lactic acid analyses at weekly intervals during the malo-lactic secondary fermentation, and S02 and color analyses of white wines at least every two weeks until the wine is bottled. Basic analyses to monitor are V.A., S02, alcohol, pH, and T.A. 

White wines are normally fermented in containers that can be partially closed so that the surface of the fermenting wine is protected from the air by a blanket of carbon dioxide. For the home winemaker, 5-gallon carboys serve admirably. An hour or so after adding S02, one adds approximately 3% of an actively fermenting pure-culture starter yeast. 

Fermentation. The juice should be inoculated with an actively fermenting culture of one of the selected strains of S. fermentati. Such yeasts may be obtained from winery supply houses as pure cultures which must be multiplied to obtain enough inoculum for the large fermentation or as dried yeast which may be added directly to the juice. Alternatively, the juice may be fermented dry with one of the standard, non-film-forming yeasts, and then the dry white wine may be inoculated... 

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